The present invention relates to an optical window or dome assembly configured for use high at high supersonic speeds and, more particularly, to an assembly which prevents excessive heating of heat sensitive components thereof, thereby preserving the optical properties thereof at high supersonic speeds. The invention further relates to a mobile platform equipped with such an assembly.
A typical guided missile is commonly made up of a number of sections, which are housed in, or connected to a generally cylindrical housing of varying radius in the longitudinal direction.
In one type of a guided missile, at the front of the missile is the guidance section which typically includes one or more sensors, such as a Forward Looking Infrared (FLIR) or video camera, and the various electronic systems which control the sensors, analyze and interpret the signals received by the sensors, and control the flight control system which positively determines the trajectory. The guidance section may also include means for receiving signals from outside of the missile and may also include means for transmitting signals from the missile.
Behind the guidance section of the missile is the warhead which is typically a hollow cylindrically shaped casing made of high strength steel. The function of the warhead is to place an explosive charge in the appropriate position at the moment of explosion, thereby maximizing the effect of the explosion on the target. Inside the hollow casing is placed the explosive and in the rear end of the warhead lies the ignition fuse which is designed to be set off at the proper moment, typically, at some pre-determined time after the warhead encounters the target. The warhead is typically made of three sections (i) a front section, or nose, which is usually in the shape of an ogive or cone; (ii) the main section which includes the explosive charge and is usually cylindrical; and (iii) the aft section which seals the explosive charge within the casing and holds the fuse.
Behind the warhead typically lies the engine which provides thrust to the missile.
Housed in and connected to the housing at the rear of the missile, and in some cases also in other locations along the missile housing, is the flight control section, including fins and foils, which are used to adjust and stabilize the trajectory of the missile during its flight to the target.
There is often a necessity for a missile or rocket to fly at high supersonic speeds. Such a necessity may arise for a number of reasons. For example, a missile fired at a moving airplane, whether from another airplane or from a fixed position on the ground, must travel at a speed greater than that of the target airplane. The distance between the launch point and the target airplane at the time of launch together with the speed of the target airplane will determine the speed at which the missile must travel. Since modem warplanes typically fly at speeds in excess of Mach 1, there is a need for missiles which fly at far greater speeds, for example Mach 4 or Mach 5. Additionally, missiles fired at stationary targets which are heavily defended by antimissile defense systems are most likely to reach the target if they fly at high supersonic speeds because this minimizes the time between detection and impact during which defensive measures may be taken.
Navigation of a guided missile to target must be conducted exclusively by a guidance system. One or more guidance systems are generally employed. Radar is one such guidance system. Radar is effective, but is subject to interference, both intentional interference deployed as defense mechanism, and accidental interference resulting from environmental conditions. Therefore, radar is often employed in conjunction with optical or electro-optical guidance systems, either of which may operate in the visible or infrared portion of the spectrum. These guidance systems are composed of a sensor or a detection system (e.g., electro-optical camera), and an analyzing system. The detection system must be onboard, although the analyzing system may be located outside the missile, for example at a base on the ground or in a platform such as an airplane which launched the missile, which communicates with the missile during flight. Alternatively, both the detection system and the analyzing system are carried on-board. This alternative, referred to as a “launch and forget” guidance system, is especially desirable in the case of missiles flying at high supersonic speeds where the time available for navigation decisions is extremely short, making communication with a remote location a practical impossibility.
The detection system must have a sensor in communication with the environment. At the same time, the sensor must be protected from the environment. For optical or electro-optical guidance systems this protection typically takes the form of an optical window or dome. These windows or domes are transparent to transmissions in a chosen range of wavelengths, while being opaque to transmissions with a wavelength outside that range. These optical windows or domes are typically coated with a shielding material which gives the window or dome the desired optical properties. As explained by D. Harris in “Materials for Infrared Windows and Domes (SPIE Optical Engineering Press, 1948), which is incorporated herein by reference, most common approaches to shielding include coating the optical window with an electrically conductive layer, covering the window with a metallic mesh, or increasing the conductivity of the material forming the window. In general, the thin electrically conductive coatings applied to the window are transparent at visible and/or infrared frequencies, but opaque to microwaves and radio waves. This makes such coatings useful in shielding sensitive electro-optical detectors against harmful electromagnetic interference (Kohin et al., SPIE Crit. Rev. CR39: 3-34(1992)). The shielding capabilities of these materials sterns from their ability to reflect and/or absorb incident radiation. In general, the greater the conductivity of the coating material, the more effective the shielding. Common coating materials are described in, for example, (i) Pellicori and Colton, Thin Solid Films 209: 109-115 (1992); (ii) Rudisill et al., Appl. Opt. 13: 2075-2080 (1974) and (iii) Bui and Hassan. Proc. SPIE 3060:2-10 (1997), all of which are incorporated herein by reference. Since the conductivity of these materials decreases with increasing temperature, they lose their shielding effectiveness when they are heated. At the same time, transmission of desired wavelengths through the shield is often diminished by heating.
Unfortunately, at high supersonic speeds (e.g., several mach), friction from the air causes heating of the optical window or dome, changing the conductivity of the coating and altering the optical properties thereof. This results in incapacitation of the detection system, either because transmissions in the chosen range of wavelengths no longer pass through the window or dome, or because interference (transmissions with a wavelength outside the chosen range) is allowed to pass through the window or dome.
There is thus a widely recognized need for, and it would be highly advantageous to have, an optical window or dome assembly which would be useable at high supersonic speeds without significant alterations in optical properties.